In order to improve the reproduction quality of a read signal read from the recording medium on which the data is recorded at high density, a patent document 1 discloses such a technology that a reproduction system which does not require a high-frequency component is realized by applying a partial response method and by intentionally providing waveform interference in a reproduction system in which an S/N ratio is increased and the amplitude of a high-frequency component is deteriorated with increasing the recording density in a linear recording direction, and that a maximum likelihood decoding method is applied by which a most probable series is estimated from a probability calculation considering the waveform interference (a technology about a so-called PRML process).    Patent document 1: Japanese Patent Application Laid. Open No. 2005-93033